1. Field of the Invention
The present invention relates to a method for measuring the speed of a vehicle relative to the ground by means of radar, using the reflection of electromagnetic waves from the roadway.
2. Description of the Related Art
The speed of a vehicle relative to the ground is conventionally measured by using the rotary speed of the vehicle wheels, and more particularly of the nondriven wheels. However, in certain cases, such as if these wheels lock during braking or in vehicles with all-wheel drive, this speed measurement may be erroneous. Hence other methods for measuring the speed have been proposed, for example utilizing the Doppler effect.
From the prior art, for instance U.S. Pat. No. 5,243,564, a device for measuring the speed of a motor vehicle is known that utilizes the modification, caused by the Doppler effect, of the frequency of an ultrasonic wave transmitted by the vehicle and reflected by the ground back to it. In this reference, it is proposed that one of the problems associated with this type of measurement, in this case the influence of the horizontal position of the vehicle on the measurement be compensated for by means of a device using the rotary speed of the wheels, measured by an antilock brake system, to determine a load transfer angle of the vehicle. However, aside from the fact that such a system is expensive because it requires equipping the vehicle with an antilock brake system, the measurement that can be obtained with it can still be incorrect. In fact, especially if an electromagnetic wave (radar wave) is employed to improve precision, difficulties are still encountered from numerous perturbation factors intrinsic to the traffic field. In particular, the wave reflected to the receiver in the vehicle may have traversed multiple courses or have been reflected by parasitic targets. As a consequence, the frequency analysis performed to extract the speed information may easily be perturbed by these conditions and lead to erroneous results, which are all the harder to detect since they are at least plausible.